Redox Flashcards
What 2 steps should you do to the metal electrode before setting up the cell?
- Rid of surface impurities by cleaning with sandpaper
- Wash away grease (e.g. from hands) off the metal with propanone
What are salt bridges made out of?
Filter paper soaked in saturated KNO3
Where should the salt bridge be, and what is its purpose?
The salt bridge should be in both solutions, but not touching either electrode. The salt bridge completes the circuit, balancing the charges
What are electrochemical cells made up of?
Two half cells joined by a wire, voltmeter and salt bridge
What is the Ecell/EMF?
The potential difference between the two cells, as shown by the voltmeter
How do you set up half cells for two different ions of the same element (e.g. Fe2+ and Fe3+)?
Use a solution containing equal amounts of 1 mol dm-3 solution for each ion. In place of the metal electrode, use platinum
Why is platinum used as an electrode?
It is inert, but electrically conductive
How can you tell which half cell will be oxidised and which reduced?
NO PRoblem
The half cell with the most Negative electrode potential will be Oxidised.
The half cell with the most Positive electrode potential will be Reduced.
What happens to the half cell being oxidised?
The metal of this half cell will be dissociating more, so more electrons and ions are being produced, and some of these electrons will flow to the other half cell
What happens to the half cell being reduced?
There will be a net movement of electrons into this half cell
What observation will be seen about the metal being oxidised? (use Zn as the example)
The favoured reaction will be Zn(s) -> Zn2+(aq) +2e-
Therefore the metal Zn(s) will become thinner
What observation will be seen about the metal being reduced? (use Cu as the example)
The favoured reaction will be Cu2+(aq) + 2e- -> Cu(s)
Therefore the metal Cu(s) will become thicker
What would be the overall equation for a Zinc and Copper electrochemical cell?
Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s)
What is standard hydrogen electrode used for?
Standard hydrogen electrodes are used as a reference to measure standard electrode potentials
What is the E0 of the standard hydrogen electrode?
0V
What are the standard conditions for E0 values?
- 298K
- 100kPa
- Concentrations of ions at 1 mol dm-3
How do you get a 1 mol dm-3 H+ ion solution for the standard hydrogen electrode?
1 mol dm-3 of HCl or 0.5 mol dm-3 of H2SO4
How do you find the standard electrode potential of a half cell?
Connect the half cell to a standard hydrogen electrode under standard conditions
What are reducing agents?
Reducing agents lose electrons and are oxisised
What are oxidising agents?
Oxidising agents gain electrons and are reduced
How do you find E0cell?
E0cell = E0reduced - E0oxidised
Why do we use standard conditions for calculating electrode potentials?
Because the half cell is affected by changes in temperature, concentration and pressure
In cell notation, what do single vertical lines show?
A state phase boundary
In cell notation, what do two vertical lines show?
The salt bridge
How do you separate ions in cell notation?
With a comma. E.g:
Pt | Fe2+, Fe3+
What does a positive E0cell value indicate?
All feasible reactions will have a positive E0cell value
Even with a standard electrode potential indicating that the reaction is feasible, what are three reasons why it may not take place?
- The reactants may have a high activation energy
- The rate of reaction may be so slow that it appears there is no reaction (e.g. rusting)
- The reaction may not take place under standard conditions
What is the relationship between E0cell and total entropy, and what does this tell you?
E0cell is directly proportional to total entropy, meaning if E0cell is positive, total entropy will also be positive, so the reaction will be feasible
What is the relationship between E0cell and lnK (equilibrium constant)?
E0cell is also directly proportional to lnK
How do we recharge batteries?
Supplying a current forces electrons to flow in the opposite way, so the reverse of the original discharge equation will occur, recharging the battery
Why are redox titrations with potassium manganate done in acidic conditions?
To prevent the formation of manganese oxide, which is a brown precipitate, so would interfere with the end point
What acid is used in titrations with potassium manganate and why?
H2SO4.
Other acids like HCl are not used, as the manganate would be involved in side reactions e.g. oxidise the chlorine
Why is an indicator not required for potassium manganate titrations?
As MnO4- is purple, and Mn2+ is colourless, so the end point of the titration is as soon as a faint pink/purple colour remains
What is the equation for the reaction between potassium manganate and iron(II) ions?
MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H20 + 5Fe3+
What is the molar ratio between MnO4- and Fe2+?
MnO4- : Fe2+
1 : 5
Why H2SO4 boiled before being added to the Fe2+ solution?
To boil away any oxygen
What is the equation for the reaction between potassium manganate and ethanedioic acid?
2MnO4- + 6H+ + 5H2C2O4 -> 2Mn2+ + 8H2O + 10CO2
What fact about the titration between potassium manganate and ethanedioic acid can lead to errors?
The reaction is very slow, so the pink colour does not immediately disappear when the initial sample manganate is added, and this could lead you to believe the titration is over prematurely
Why does rate of reaction of potassium manganate and ethanedioic acid speed up as the titration proceeds?
As Mn2+ (which is produced) acts as a catalyst for the reaction
What is the test for iodide ions, and what is the positive result?
Add silver nitrate
Pale yellow precipitate
